Homework Helpers: Physics

5 Electric Charges, Forces, and Fields

Lesson 5–3: Methods for Charging Objects

You have probably employed several methods to charge objects in the past. Did you ever rub a balloon on your head and then stick it to a wall? If so, you have performed two out of the three methods for charging objects that we will be learning in this lesson. Before we get to these methods, however, we will go over some preliminary information that relates to these methods of charging.

If you think of electrons as the carriers of flowing charge, you may realize that this charge will not flow with equal ease across all surfaces. Some materials, called conductors, allow electrons to flow freely through them. Metals are good conductors, which is why we use them for wiring and lightning rods. Other materials, called insulators, do not allow electrons to flow easily through them. Rubber and plastic are good insulators, and we use these materials to block the flow of electricity. For example, wires are covered with rubber insulation.

One important difference between insulators and conductors is what happens to excess charges on them. If you give a metallic object an excess negative charge, the electrons will spread out over the surface of the object, trying to maximize the distance between electrons. This occurs because the electrons are all like charges, so they repel each other. If the object is a uniform sphere, the charge spreads across the surface of the sphere until it is uniformly distributed. If the conductor doesn’t have a uniform shape, charges tend to build up on the surface of points and protrusions.

For insulators, the charges aren’t free to move. If you charge a particular area of the insulator, the charges can’t migrate freely through or across it, so they tend to stay in that area until they are given an opportunity to leave. A balloon is made of rubber, a good insulator, so if you rub the balloon in your hair, it will strip excess electrons from your hair, leaving an area of the balloon with a net negative charge. These electrons will stay on that particular area unless a conductive path is provided to allow them to escape.

Dry air is a good insulator, but moist or humid air is a fairly good conductor. When charged objects are surrounded by dry air, they will hold on to their excess charge longer than if they were surrounded by humid air. One reason why static cling occurs in the dryer is because the air inside the dryer becomes (not surprisingly) very dry. Charged clothing items will retain a charge for longer and stick to another item.

People with straight hair suffer more “bad hair days” in dry weather, because their individual hairs will obtain like charges and repel each other. The result is what is sometimes called “fly-away” hair. If you are not familiar with this phenomenon, you may have seen it at the park instead. When my daughter slides down a plastic slide on a dry day, much of her hair starts to stand straight up. The reason for this effect can be understood better by looking at an instrument called an electroscope, which is used to determine the type of charge on an object.

Figure 5.4

Figure 5.5

The important part of an electroscope to watch is the vanes. The vanes are usually made of some light metal foil. The metal foil is a good conductor, and it is attached to the knob. When a charged object comes near to, or touches, the knob, the vanes become charged. When the vanes are not charged, they hang together, but when they both obtain a like charge, they repel each other and hang more apart.

This is just what happens to some hair on dry days. The individual strands of hair obtain like charges and repel each other.

An electroscope can be used to determine the type of charge on an object when you don’t know whether the object is positive or negative. Let’s start off with the example of rubbing the balloon in your hair. I told you earlier in this lesson that this method will result in a balloon with an excess of electrons and a net negative charge on an area of the balloon’s surface. If you bring this negatively charged area of the balloon close to a neutral electroscope, you will notice that the vanes of the electroscope move slightly apart, before the balloon even touches the knob. This happens through our first method of charging, called polarization.

Polarization is the process of inducing a temporary separation of charges in a neutral object by bringing it in close proximity to a charged object. Think of the knob and vanes of the electroscope. They are made of metal, and some of the electrons in metals are free to move. When you bring a negatively charged balloon near the knob of the electroscope, the negative area of the balloon will repel the electrons in the knob. Because the electrons in the knob are free to move, some will migrate to the vanes in order to get away from the excess electrons on the balloon. As these electrons gather on the vanes, they repel each other, causing the vanes to move apart.

What makes this method of charging temporary is that if you move the balloon away from the electroscope, having never actually touched the knob with it, the excess electrons from the vanes will now redistribute themselves over the entire conductive surface, returning the electroscope to the uncharged state.

Now, let’s go over the method of charging called conduction. Conduction is the process of charging a neutral object by bringing it in contact with a charged object. Let’s suppose that we now take the area of the balloon with the excess electrons and actually touch the knob of the electroscope. Remember: The excess electrons on the balloon want to get away from each other. It is only the fact that the balloon is made of an insulator that prevents the electrons from spreading out. Now the electrons have come in contact with the conductive metal of the electroscope, and many of them will take this opportunity to spread out on to the surface of the electroscope. The vanes of the electroscope spread out again, indicating that they have obtained a charge. Now, even when we remove the balloon, the vanes of the electroscope stay open because the electroscope has a residual charge.

Before we go over our last method of charging, I want to discharge the electroscope and return it to its neutral state. I can accomplish this through grounding. Grounding is the process of providing a conductive pathway between a charged object and the ground, allowing the charged object to become neutral. In our example, the electroscope had an excess of electrons. If I touch the electroscope with my finger, the excess electrons can travel onto my finger, through my body, and into the ground. Grounding my electroscope will make it neutral again.

Induction is the process of charging a neutral object without ever having it touch another charged object. Induction involves bringing a charged object close enough to a neutral object to polarize it, and then grounding the neutral object in order to allow a charge to build up on it. Finally, the ground is removed before the original charged object is taken away.

So, unlike conduction, induction does not involve direct contact between the original charged object and the original neutral one. Another important difference between conduction and induction is that whereas conduction results in the neutral object taking on the same type of charge as the charged object, induction results in the neutral object taking on the opposite type of charge as the charged object.

Let’s review before moving on.

Lesson 5–3 Review

1. _______________ is the process of charging an object by bringing it into direct contact with another charged object.

2. _______________ is the process of providing a conductive path between a charged object or a circuit to the ground.

3. _______________ is the process of inducing a temporary separation of charges in a neutral object by bringing it in close proximity to a charged object.

4. Can you tell the type of charge (positive or negative) on a charged object by bringing it near a discharged (neutral) electroscope and seeing how the vanes react?